Cellular encapsulation is a technique that consists in immobilizing cells or clusters of cells in microcapsules so as to protect them from immune system attacks after being transplanted. The capsules need to be sufficiently porous to allow entry of molecules of low molecular weight that are essential for the metabolism of the encapsulated cells, such as nutrient molecules, oxygen, etc., while simultaneously preventing entry of substances of high molecular weight, such as antibodies or cells of the immune system. This selective permeability of the capsules is thus designed to prevent direct contact between the encapsulated cells from the donor and cells of the immune system of the recipient of the transplant, thereby making it possible to limit the doses of immunodepressive treatment used with transplantation, since said immunodepressive treatment presents severe side effects. In addition to their selective permeability, the capsules that are produced must be biocompatible, mechanically strong, and of size that matches the article to be encapsulated.
Amongst the multiple applications of encapsulation, mention can be made of islets of Langerhans, clusters of fragile cells that are situated in the pancreas and that are constituted by several types of cell including β cells that regulate glycemia in the body by producing insulin. Encapsulating these islets is an alternative to conventional cell therapies (e.g. pancreas or islet transplantation) used for treating insulin-dependent diabetes, an auto-immune disease in which the immune system destroys its own insulin-producing β cells.
The main known methods of encapsulation make use of either:                a coaxial jet of air or liquid, the resulting capsules having size that lies in the range 400 micrometers (μm) to 800 μm (although the mean size of the capsules produced lies closer to the range 600 μm to 800 μm than to 400 μm: see Zimmermann, “Fabrication of homogeneously cross-linked, functional alginate microcapsules validated by NRM-, CLSM-, and AFM-imaging”, Biomaterials, 2003, 24: pp. 2083-2096; or        a potential difference, which is the encapsulation technique in the most widespread use when the priority is to reduce the size of the capsules (capsule size then lying in the range 200 μm to 800 μm); see Goosen, “Electrostatic droplet generation for encapsulation of somatic tissue: assessment of high-voltage power supply”, Biotechnol. Prog., 1997, 13, pp. 497-502; or else        a vibration technique that presents the drawback of sometimes being limited by the viscosities of the solutions being used; see Seifert, “Production of small, monodispersed alginate beads for cell immobilization”, Biotechnol. Prog., 1997, 13, pp. 562-568.        
Those techniques known in the prior art include certain drawbacks:                the size of the capsule is not necessarily appropriate for the size of the cells/islets that are to be encapsulated;        the dispersion of capsule size increases with decreasing drop size; and        the capsules produced are not necessarily spherical, thereby leading to a lack of reproducibility.        
In addition to those problems, most present encapsulation techniques do not provide any way of controlling the number of cells or cell clusters that are contained in each droplet. The number of cells or clusters that are encapsulated is determined only statistically by adjusting the concentration of the suspension of cells (or clusters) in the polymer solution acting as the encapsulation matrix (this concentration depends on the size of the particles for encapsulation and on the size desired for the capsules). Conventional systems thus produce a very large number of empty capsules together with capsules containing varying numbers of cells or clusters.
The main drawback of empty capsules is to increase the total cell volume that needs to be transplanted and to prevent them being transplanted into zones that would be particularly suitable for tissue revascularization, which is essential to avoid necrosis of the encapsulated cells, since they need to be close to the blood network in order to be fed with nutrients and oxygen. For example, for treatment of type 1 diabetes, a reduction in the total volume of capsules for transplanting would make it possible to implant encapsulated islets in the liver or the spleen, regions that are more favorable for revascularization than the peritoneal cavity where capsules are conventionally implanted for questions of steric hindrance.
When the capsules contain varying numbers of cells or clusters, there is a risk of the cells or clusters projecting from the surface of the capsule, thereby running the risk of triggering an immune reaction in which the graft is rejected. This problem is made worse when the size of the capsule is a close fit to the size of the article to be encapsulated.
To solve the drawbacks of the above-mentioned conventional techniques, encapsulation devices and methods have been proposed that are based on microfluidic techniques. See for example:                Workman et al., “Microfluidic chip-based synthesis of alginate microspheres for encapsulation of immortalized human cells”, Biomicrofluidics, 2007, 1. That article describes a method of fabricating capsules having a diameter lying in the range 80 μm to 400 μm approximately, by means of a so-called “hydrodynamic focusing” effect; and        Sugiura et al., “Size control of calcium alginate beads containing liver cells using micronozzle array”, Biomaterials, 2005, 26: pp. 3327-3331. In the method described in that publication, capsules of size lying in the range 50 μm to 200 μm are formed by injecting alginate into an oily phase through “micronozzles” having a diameter of 30 μm.        
Those techniques provide better control over capsule size, and said size is reduced compared with using conventional techniques; however the number of particles (cells or clusters) per capsule remains highly variable.
At present, the only technique known to the inventors that enables isolated particles to be encapsulated relies on the combined use of microfluidics and of optical tweezers enabling a cell contained in the aqueous phase to be moved towards the water/oil interface at a T-junction. This technique also presents the advantage of producing a capsule of size that is of the same order of magnitude as the size of the encapsulated cell. Nevertheless, it does not make it possible to achieve encapsulation rates that are sufficient to make application thereof viable outside the field of scientific research.